Introduction
When manufacturing chemical and petroleum equipment, in order to save expensive nickel, steel is often welded to nickel and alloys.
The main problems of welding
When welding, the main components in the weld are iron and nickel, which are capable of infinite mutual solubility and do not form intermetallic compounds. In general, the nickel content in the weld is relatively high, so in the fusion zone of the welded joint, no diffusion layer is formed. The main problem with welding is the tendency to produce porosity and hot cracks in the weld.
1.Porosity
Steel and nickel and its alloys when welding, the main factors affecting the formation of porosity in the weld is oxygen, nickel and other alloying elements.
① The effect of oxygen. Welding, the liquid metal may dissolve more oxygen, and oxygen at high temperatures and nickel oxidation, the formation of NiO, NiO can react with hydrogen and carbon in the liquid metal to generate water vapor and carbon monoxide in the molten pool solidification, such as too late to escape, residual in the weld on the formation of porosity. In pure nickel and Q235-A submerged arc welding of iron and nickel weld, in the case of nitrogen and hydrogen content does not change much, the higher the oxygen content in the weld, the higher the number of pores in the weld.
② The effect of nickel. In the iron-nickel weld, the solubility of oxygen in iron and nickel is different, the solubility of oxygen in liquid nickel is greater than that in liquid iron, while the solubility of oxygen in solid nickel is smaller than that in solid iron, therefore, the solubility of oxygen in the nickel crystallization of the sudden change is more pronounced than that in the iron crystallization of the sudden change. Therefore, the tendency of porosity in the weld when Ni is 15%~30% is small, and when the Ni content is large, the tendency of porosity is further increased to 60%~90%, and the amount of dissolved steel is bound to decrease, thus causing the tendency of forming porosity to become larger.
③ The influence of other alloying elements. When the iron-nickel weld contains manganese, chromium, molybdenum, aluminum, titanium and other alloying elements or in line with the alloying, can improve the weld anti-porosity, this is due to manganese, titanium and aluminum, etc. have the role of deoxygenation, while chromium and molybdenum to improve the weld solubility in the solid metal. So nickel and 1Cr18Ni9Ti stainless steel weld anti-porosity than nickel and Q235-A steel weld. Aluminum and titanium can also fix nitrogen in stable compounds, which can also improve the weld anti-porosity.
2. Thermal cracking
Steel and nickel and its alloys in the weld, the main reason for thermal cracking is that, due to high nickel weld with dendritic organization, in the edge of the coarse grains, concentrated in a number of low melting point co-crystals, thus weakening the connection between the grains, reducing the weld metal crack resistance. In addition, the nickel content of the weld metal is too high for the weld metal to produce thermal cracking has a significant impact in the iron-nickel weld, oxygen, sulfur, phosphorus and other impurities on the weld thermal cracking tendency also has a great impact.
When using oxygen-free flux, due to the reduction in the quality of oxygen, sulfur, phosphorus and other harmful impurities in the weld, especially the decline in the oxygen content, so that the amount of cracking is greatly reduced. Because the molten pool crystallization, oxygen and nickel can form Ni + NiO eutectic, eutectic temperature of 1438 ℃, and oxygen can also strengthen the harmful effects of sulfur. So when the oxygen content in the weld is high, the tendency of thermal cracking is larger.
Mn, Cr, Mo, Ti, Nb and other alloying elements, can improve the crack resistance of the weld metal.Mn, Cr, Mo, Ti, Nb are metamorphic agent, can refine the weld organization, and can disrupt the direction of its crystallization.Al, Ti is also a strong deoxidizing agent, can reduce the amount of oxygen in the weld.Mn can form refractory compounds with S, MnS, which reduces the harmful effects of sulfur.
Mechanical properties of welded joints
The mechanical properties of iron-nickel welding joints are related to the fill metal materials and welding parameters. When welding pure nickel and low carbon steel, when the Ni equivalent in the weld is less than 30%, under the rapid cooling of the weld, a martensite structure will appear in the weld, causing the plasticity and toughness of the joint to drop sharply. Therefore, in order to obtain better plasticity and toughness of the joint, the Ni equivalent in the iron-nickel weld should be greater than 30%
Post time: Mar-10-2025